The primary goal of this research project is to evaluate the functional role of the auditory thalamo-cortical system in hearing behavior in primates. A joint approach including animal psychophysical procedures, and chronic electrophysiological techniques will be employed. Hearing behavior will be evaluated in a simple reaction-time task which has been demonstrated to yield precise measures of threshold and suprathreshold hearing. Unit activity will be recorded from higher central auditory structures, particularly auditory cortex, simultaneously with the evaluation of hearing performance. Primarily we will attempt to describe the response characteristics of auditory cortex activity in relationship to intensity of acoustic stimulation. The influence of behavioral state on intensity functions of cortical cells will be examined. The relationship between cellular function and a behavioral measure of loudness will also be studied. Neural mechanisms underlying unit intensity function shape will be examined.